1. Field of the Invention
The present invention relates generally to monitoring liquid storage tanks for corrosion. More particularly, the present invention relates to a novel apparatus and method for the in situ monitoring such tanks for thinning (due to corrosion) of the tank bottom, and optionally for uneven settling of the tank, using ultrasonic transducers mounted on a mobile robotic device for use inside liquid filled tanks.
2. Description of the Related Art.
With respect to storage tanks.
Large metal storage tanks are used to store a variety of liquids, including especially water, petroleum, and petroleum products. These tanks, which are commonly made from non-stainless steel plates, are subject to corrosion and eventual failure over time. Even tanks that are intended to store only petroleum and petroleum products almost inevitably will have a substantial amount of water in them, greatly aggravating their tendency to oxidize. Thus, it is necessary to periodically inspect these tanks, and to make repairs before such failure. However, because of the enormous size of these tanks, it is expensive and inconvenient to empty a tank each time it is to be inspected.
Typical tanks for petroleum products are made from 1/4" to 1/2" thick steel plates welded together. They are commonly hundreds of feet tall and hundreds of feet in diameter. They are usually built above ground, but may also be made at least partially below ground. If such a tank should fail while filled with product, the environmental damage would be staggering.
Complicating the inspection process is the presence of sediment in the tanks. At the bottom of most liquid storage tanks there is a layer of sediment including rust, dirt, debris, petroleum solids, etc. This layer may be anywhere from several millimeters to several feet thick. An effective tank inspection system must either be able to see through this sediment, or must be able to displace the sediment to allow for direct inspection.
Bilges of large ships (e.g., aircraft carriers, tankers) likewise must be inspected periodically for corrosion. Many of the same concerns are raised for the inspection of bilges as are raised for the inspection of tanks.
With respect to corrosion monitoring systems.
Available systems for monitoring liquid filled tanks for corrosion suffer from any of several common shortcomings.
One problem that has not been satisfactorily addressed by the art has been the difficulty of using ultrasonic probes to obtain reflection signals from both the top and bottom surfaces of the base plate of a tank. The sediment layer described above scatters ultrasonic waves, dramatically attenuating these waves, and thereby decreasing their ability to penetrate the bottom steel plate. Moreover, ultrasonic waves scattered from the sediment are a source of noise that present systems do not adequately address.
U.S. Pat. No. 5,205,174, issued Apr. 27, 1993 to Silverman et al. (Silverman '174) is representative. This patent teaches the use of ultrasonic transducers for inspecting the bottom surfaces of storage tanks. This system teaches the use of very high frequency ultrasonic pulses (about 15.4 MHz, with a wavelength of 0.0156'). Such high frequency pulses will be so diminished in amplitude that signal detection would be problematic at best. This is particularly true if the ultrasonic pulse is scattered by plate surface that has been roughened by corrosion and attenuated by a layer of sediment. Silverman '174 attempts to mitigate the former problem by including a cleaning system in the apparatus, to remove sediment from the bottom as the apparatus moves along. In principle, this system would work by scrubbing and vacuuming the base free of sediment, irrigating the base with a stream of clean fluid to remove any remaining sediment, and filtering out the sediment so that it is not returned to the tank. At best, this is a very complicated system that does not address the problem of scattering by a surface that has been roughened by corrosion.